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Josie Sandercock <[log in to unmask]> recommended:
Assman et al, Lancet 2000 - Subgroup analysis and other (mis)uses of
baseline characteristics (25 March, pp1064-1069)


This paper does indeed clarify the discussion, and spells out that its
authors found in a survey of the medical literature numerous examples of
poor statistical practice.  These included not just the use of baseline
data, but also subgroup analysis, the "exaggerated claims of treatment
effects arising from post-hoc emphases across multiple analyses" and
"lack of clarity" in explaining covariate-adjusted analyses especially
when interactions occurred.

Assman's recommendation on baseline comparisons coincides exactly with
mine: "Although reports should show in approriate detail the types of
patient included, the baseline comparisons across treatments *need not be
so extensive* [as was being published].

However, Assman's comment on covariate adjustment shows "lack of clarity"
verging on delphic.  "Adjustment for baseline factors with treatment
imbalances is unimportant, unless such factors relate to outcome." In
other words, it is *essential* to test covariates in the model.  If you
thought they could not affect the outcome, why measure or record them? On
the other hand, when reporting results, *report the results*, with
suitable emphasis that relates to clinical importance, not abstract P
values.

I am pleased, therefore, to confirm my sanity, and to suggest that medics
who dabble in statistics are about as useful as statisticians who offer
ad hoc diagnoses of medical conditions.

In contrast to this morning's summary, and mirroring the advice offered
in the Peto reference (Lancet. 1999 Jul 3;354(9172):73. quoted by Paul Seed),
the received wisdom should be, *do the analysis* and *interpret the results*.
Then choose what and how to report.  And that is not an invitation to
cherry pick whatever results suited your prejudice, but a recognition
that any report is produced by a process of selection and ordering.

R Allan Reese